Teams hold onto quarterbacks too long, even if they probably know it's a bad idea. The fear of the unknown at quarterback leads to poor decisions. Too often franchises believe it's better to have a proven mediocre QB than gamble on a new one and end up with utter incompetence. Bottoming out at QB gets head coaches and general managers fired.
That's why the Minnesota Vikings' offseason was unusual. It was a reason everyone gave up on them. The Vikings moved on from Kirk Cousins, reportedly refusing to increase their offer to their QB of six seasons. The Atlanta Falcons came in with a better offer, leaving the Vikings without a quarterback at all.